The invention relates to a process for preparing phthalic anhydride by catalytic partial oxidation of o-xylene and/or naphthalene with a gas comprising oxygen in a plant comprising two or more cooled reaction zones.
In chemical process technology, a multitude of partial oxidation reactions of fluid, i.e. gaseous, liquid or gaseous/liquid, reaction mixtures are known, which are carried out in the presence of heterogeneous particulate catalysts. Such reactions are generally exothermic, frequently strongly exothermic.
Phthalic anhydride is prepared on the industrial scale by partial oxidation of o-xylene and/or naphthalene with a gas comprising oxygen, frequently air, over heterogeneous, especially supported, catalysts. The reaction enthalpy for the oxidation of o-xylene is 1110 kJ/mol and, for naphthalene, 1792 kg/mol of phthalic anhydride formed, i.e. the reaction is strongly exothermic. For inexpensive production with high reactor throughputs, a high loading of the air with reactant is required; these gas mixtures are frequently then ignitable.
To reduce the proportions of undesired intermediates, byproducts and also of reactant in the process gas, which present problems in the product removal from the gas stream and the further workup, it is usually necessary to attain reactant conversions close to 100 mol %.
As in the case of other partial oxidations too, it is typical that, in the preparation of phthalic anhydride, the approach of the reactant conversion to 100% is associated with an increasing proportional rate of total oxidation to carbon monoxide and carbon dioxide. It is also possible at too low a reaction rate for by-products to form, especially phthalide in the case of the synthesis of phthalic anhydride, by which the product quality is impaired. For the reasons mentioned, a high conversion and thus a perceptible yield loss have to be accepted according to the prior art to obtain on-spec product.
Owing to the aging of the catalyst with increasing operating time, it is necessary for a uniform conversion to raise the temperature in the reactor, as a result of which even higher yield losses have to be accepted according to the prior art.
To a certain degree, the yield can be improved by the utilization of an uncooled postreactor in which a limited residual conversion takes place at reduced temperature, as a result of which especially compliance with the product specification is enabled. At the inlet and outlet of the postreactor, the process gas mixture still comprises a very large amount of organic substance and also free oxygen and is generally ignitable. To limit the temperature rise, the conversion of the postreaction has to be strictly limited to avoid runaway of the reaction.
DE-A 101 44 857 discloses a reactor arrangement in which a main reactor designed as a tube bundle reactor, a cooling stage and also a postreactor are arranged in a single casing, the postreactor being an uncooled shaft reactor. This arrangement in one casing keeps the gas volume between main reaction zone and postreaction zone small, with the advantage that the risk of ignition is greatly reduced there. In this way, it is intended that an advantageous yield should be realizable even at loadings of over 100 g of o-xylene per m3 (STP) in the reactant mixture. With the proposed operating mode, an adiabatic reaction in the postreactor, the possible conversion there is, however, greatly restricted for the reasons already mentioned above, so that neither a significant yield improvement nor in particular any prolonging of the catalyst lifetime is possible in this way.
DE-A 40 13 051 discloses a process for preparing phthalic anhydride using a tube bundle reactor comprising at least two successive reaction zones in flow direction with separate salt bath cooling, the salt bath temperature in the first reaction zone being kept from 2 to 20° higher than the salt bath temperature in the remaining reaction zones. This process achieves an improved yield of phthalic anhydride, since the temperature is adjusted better to the reaction kinetics. In spite of this, high yield losses remain owing to the high temperature level in both cooling zones, at which the required high conversion of approx. 99% mol % can be achieved.